Responding officers and firefighters performed CPR on the girl as they rushed her to a nearby hospital, where she was in "extremely critical condition" and on life support, Hill said.
"Somehow she was playing on this treadmill, and there's a cord that hangs under the console - it's kind of a loop," Hill said.
"Either she slipped or put her head in the loop, but it acted like a noose, and she was obviously unable to get herself off of it."
Hill said former heavyweight champion Tyson had been in Las Vegas but flew to Phoenix immediately after learning of the accident.
Tyson could not immediately be reached for comment. Calls to three of his representatives were either not returned or not answered.
Hill said everything in the investigation pointed to an accident. "There's nothing in the investigation that revealed anything suspicious," he said.
He added that responding to calls involving children is an officer's most difficult duty.
"Those are the things that stay with you in your career," he said. "We always home for a miracle - not to have the worst happen to a child."